During the fabrication of a semiconductor integrated circuit, thermal processing steps for performing film-formation, oxidation, or diffusion on a surface of a semiconductor wafer and pattern-etching steps are generally repeated. When wafers of, for example, an 8-inch size are to be subjected to thermal processing, a vertical batch type of thermal processing apparatus is mainly used in the art because it is capable of subjecting a large number of wafers to thermal processing at the same time. An important point with this type of thermal processing is good control over the uniformity of temperature within the wafer surface, from the viewpoint of improving uniformity of the characteristics of the completed circuits, and hence the yield thereof.
Concomitant with the increasing integration and decreasing size of circuits, the wafer size is also increasing, so that the use of wafers such as those of 12 inches in size is being investigated.
If such wafers increase in size from 8 inches to 12 inches (approximately 30 cm), the self-weight of each wafer is multiplied to approximately 2.5 to 3 times that of an 8-inch wafer, and moreover considerations of thermal uniformity within the wafer surface make it difficult for a prior-art batch type of vertical thermal processing apparatus to cope therewith. In other words, one result of the multiplication of the self-weight of the wafer as described above is that a wafer boat for holding a large number of wafers cannot cope from the strength point of view; alternatively, because the surface area of the wafer has increased due to the increase in diameter, a method that relies on heating from the sides of wafers that are arrayed at a predetermined pitch will find it difficult to heat the wafer surfaces uniformly.
To solve the above problems, there have been various proposals for a single-wafer type of thermal processing apparatus for processing one wafer at a time, wherein a wafer that is supported or mounted on a wafer holder is heated by a halogen lamp or resistance heater of the apparatus, disposed below the wafer holder. However, if such a prior-art single-wafer type of thermal processing apparatus is used to heat a larger-diameter wafer, it is fairly difficult to achieve good uniformity of the surface temperature of the wafer, with the amount of heat generated per unit surface area by a heater of a current type, and it can not be said that a conventional apparatus is sufficient therefor.
The present invention was devised in order to solve the above problems. A main objective of the present invention is to provide a single-wafer type of thermal processing apparatus that is capable of improving the uniformity of temperature within the surface of an object to be processed, such as a wafer.